Heated food service shelf for warming cookies and the like

ABSTRACT

A heating element assembly in the form of a heating shelf and a method of manufacturing heating shelf assemblies. The heating shelf may be used in display cabinets to heat ready made foods such as cookies, muffins, donuts, pizza, sandwiches and the like. The preferred heating shelf includes thermochromic materials, or an LED indicator, which provide a visual indica of shelf temperature. The preferred heating shelf provides intimate contact with the heated food products, thus optimizing heat transfer between the heating shelf and the food products. Optionally provided, varied surface watt density in the heating shelf allows for accurate heat placement such that the food products can be evenly warmed while avoiding over warming. In another embodiment, the heating shelf includes two resistance heating elements. The first heating element is a temperature booster used for defrosting and heating, while the second heating element is a maintenance heater to maintain heated food at a serving temperature.

FIELD OF THE INVENTION

[0001] This invention relates to electrical resistance heating elements,and more particularly to formable thermoplastic laminate heating elementassemblies.

BACKGROUND OF THE INVENTION

[0002] Methods for providing reformable heating element assemblies aredescribed in Applicant's co-pending application Ser. No. 09/642,215,herein incorporated in its entirety by reference.

[0003] In the food service industry, display cabinets are commonly usedto display food products for retail sale. As an example, manyconvenience stores have display cabinets that may feature varied foodproducts such as donuts, muffins, cookies and the like. Heated foodservice cabinets are also used in nursing homes and hospitals and infood service applications on board airliners and cruise ships. Oftentimes, these cabinets are fitted with heated shelves, which keep thefoods warmed to desired serving temperatures. The present method forapplying heat to shelving is to attach tubular elements to a sheet metalframework that is attached to the bottom side of a shelf. The sheetmetal framework provides a means of electrical enclosure, preventingexposure to live electrical parts. However, the resulting heatable shelfassemblies average approximately 2 inches in thickness, thus providingan inefficient use of limited cabinet space. Further, such assembliesare expensive to manufacture, distribute and maintain.

[0004] Electrically heated steel shelves may also pose significantsafety risks to food service workers and consumers. Because heated steelshelves typically lack visible features to indicate the presence ofheat, workers and consumers are susceptible to burn injuries as theyremove foods from the heated steel shelves. Moreover, humans may beexposed to significant electrical hazards through contact with theelectrically charged metal shelves.

[0005] Therefore, improved apparatus and methods for heated cabinetshelving are desirable. The ideal heating shelf would eliminate the riskof electrical hazard by insulating the user from direct contact withresistance heating elements. The preferred shelf design would alsoinclude one or more visible features that change with heat, to provide areadily perceptible heat indicia. In addition, the preferred heatingshelf would include multiple resistance heating elements to provide bothtemperature boosting for initial heating, and maintenance heating formaintaining heated foods at a serving temperature. The preferred designwould also be adaptable to for use with existing cabinet designs, whileproviding for improved utilization of existing cabinet space. Finally,the improved heatable shelf design would be cost effective to produceand operate.

SUMMARY OF THE INVENTION

[0006] The present invention provides a heating element assembly in theform of a heating shelf and a method of manufacturing heating shelfassemblies. The heating shelf may be used in existing food servicetransport and display cabinets and shelves for controlled heating ofready made food products such as cookies, muffins, donuts, pizza,sandwiches and the like. The preferred heating shelf optionally includesthermochromic materials (i.e, the materials change color withtemperature), or lighted displays, such as an LED warning light, thusproviding a visual indication of heating shelf temperature. Otherfeatures may include varied surface watt density for accurate heatplacement and multiple resistance elements for initial temperatureboosting and temperature maintenance.

[0007] The present invention as described above provides severalbenefits. One or more intricate resistance circuit paths of one or moreresistance heating materials, such as NiCr wire, graphite scrim,conductive polymers etc., may be laminated between thermoplastic sheets,wherein the planar resistance heating element may then be reformed, asby thermoforming, drawing, or moldings, with the laminated structure toprovide heat on one or more heat planes.

[0008] These heating structures provide intimate contact between thecontents of the heating structures and the heat source, therebyproviding inherent energy consumption advantages as well as the abilityto intimately locate secondary devices such as thermistors, sensors,thermocouples, RTDs, etcetera, in proximity to the contents being heatedor conditions being observed or recorded.

[0009] The heating element assembly also allows for an infinite numberof circuit path shapes, and designs, allowing the circuit path tocorrespond to the general shape of a desired end product utilizing theheating element assembly. The heating element assembly may be folded tooccupy a predefined space in an end product and to provide heat in morethan one plane, thermoformed into a desired three dimensional heatedplane, or stamped or die cut into a predetermined flat shape which may,then, be folded or thermoformed into a desired three dimensional heatedshape. The heating element assembly thereby emulates well known sheetmetal processing or known plastic forming processes and techniques.

[0010] The heating element assembly according to the present inventionmay also be over molded in a molding process whereby the resistanceheating element is energized to soften the thermoplastic sheets and theheating element assembly is over molded with a thermoplastic to form adetailed molded structure. The energizing and overmolding steps may betimed such that the thermoplastic sheets and over molded thermoplasticform a substantially homogenous structure accurately capturing andpositioning the resistance heating element within the structure.Alternatively, the heating element assembly may soften during mold flowwithout additional energizing.

[0011] In addition, thermochromic materials, or lighted displays, suchas colored LEDs and thermometers, may be integrally formed with theheating shelf to provide a visual indicia of shelf temperatures.

[0012] In another embodiment of the present invention, a sheet ofheating element assemblies comprises a first thermoplastic sheet, asecond thermoplastic sheet affixed to the first thermoplastic sheet, anda sheet of resistance heating elements secured between and to the firstand second thermoplastic sheets. The sheet of resistance heatingelements includes a supporting substrate having a first surface thereonand a plurality spaced circuit paths, each of the circuit pathscomprising at least one electrical resistance heating material attachedto the supporting substrate wherein at least one of the circuit pathshas terminal end portions.

[0013] The sheet of heating element assemblies of this embodimentprovides several benefits. The sheet may be inexpensively andefficiently produced using mass production techniques. The sheet may becollected into a roll, allowing the later separation and use ofindividual heating element assemblies or group of heated elementassemblies as described above. The sheet, may be further, oralternatively, processed using various secondary fabrication techniques,such as stamping, die cutting, or overmolding.

[0014] The above and other features of the present invention will bebetter understood from the following detailed description of thepreferred embodiments of the invention which is provided in connectionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The accompanying drawings illustrate preferred embodiments of theinvention, as well as other information pertinent to the disclosure, inwhich:

[0016]FIG. 1 is a top plan view of a pair of resistance wires disposedin predetermined circuit paths according to an exemplary embodiment ofthe invention;

[0017]FIG. 2 is a front perspective view of a preferred programmablesewing machine and computer for manufacturing resistance heatingelements;

[0018]FIG. 3 is an isometric view of a first embodiment of the heatingelement assembly according to the invention, with a portion of a toplaminate surface removed to reveal a portion of the resistance heatingelement;

[0019]FIG. 4 is a partial cross-sectional elevation view of the heatingelement assembly shown in FIG. 3, taken along line 4-4;

[0020]FIG. 5 is a partial cross-sectional view of a multi-layeredheating element assembly according to the invention;

[0021]FIG. 6 is a diagram of an exemplary method of manufacturing asheet of heated element assemblies according to the invention;

[0022]FIG. 7 is a diagram of a sheet of resistance heating elementsshown in partial view according to the invention;

[0023]FIG. 8 is a top plan view of a resistance heating element assemblywherein the laminated structure has been cut to form a profile for aheating container which may be folded to form a three dimensional heaterassembly;

[0024]FIG. 9 is a top plan view of a heating element assembly includingthe resistance heating element of FIG. 8 wherein a portion the toplaminated surface has been removed to show the resistance heatingelement, before being formed into a final configuration; and

[0025]FIG. 10 is as a performance graph of a heating assembly accordingto the invention, in which the heating assembly is used to heatprepackaged, baked cookies.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0026] The present invention provides a thermoplastic laminate heatingelement assembly including resistance heating elements, in the form of aheating shelf. As used herein, the following terms are defined:

[0027] “Laminate” means to unite, for example, layers of laminate viabonding them together, usually with heat, pressure and/or adhesive. Itnormally is used to refer to flat sheets, but also can include rods andtubes. The term refers to a product made by such bonding;

[0028] “Serpentine Path” means a path which has one or more curves forincreasing the amount of electrical resistance material in a givenvolume of polymeric matrix, for example, for controlling the thermalexpansion of the element;

[0029] “Melting Temperature” means the point at which a fusiblesubstance begins to melt;

[0030] “Melting Temperature Range” means the temperature range overwhich a fusible substance starts to melt and then becomes a liquid orsemi-liquid;

[0031] “Degradation Temperature” means the temperature at which athermoplastic begins to permanently lose its mechanical or physicalproperties because of thermal damage to the polymer's molecular chains;

[0032] “Evacuating” means reducing air or trapped air bubbles by, forexample, vacuum or pressurized inert gas, such as argon, or by bubblingthe gas through a liquid polymer.

[0033] “Fusion Bond” means the bond between two fusible membersintegrally joined, whereby the polymer molecules of one member mix withthe molecules of the other. A Fusion Bond can occur, even in the absenceof any direct or chemical bond between individual polymer chainscontained within said members;

[0034] “Fused” means the physical flowing of a material, such asceramic, glass, metal or polymer, hot or cold, caused by heat, pressureor both;

[0035] “Electrofused” means to cause a portion of a fusible material toflow and fuse by resistance heating;

[0036] “Stress Relief” means reducing internal stresses in a fusiblematerial by raising the temperature of the material or material portionabove its stress relief temperature, but preferably below its HeatDeflection Temperature; and

[0037] “Flap” or “Flap Portion” means a portion of an element which canbe folded without damaging the element structure.

RESISTANCE HEATING ELEMENT

[0038] With reference to FIGS. 1-9, there is shown a first embodiment ofa resistance heating element 10, preferably having about 50-95% of thesurface area of the heated shelf. The preferred resistance heatingelement 10 may include a regulating device for controlling electriccurrent. Such a device can include, for example, a thermistor, athermocouple, or a RTD, for preventing overheating of the polymericmaterials disclosed in this invention. The resistance heating elements10 of this invention can take on any number of shapes and sizes,including squares, ovals, irregular circumference shapes, tubes, cupshapes and container shapes. Sizes can range from less than one inchsquare to 21 in.×26 in. with a single sewing operation, and greatersizes can be available if multiple elements are joined together. Greatersizes are also available with roll or continuous element forms.

[0039] As shown in FIG. 1, a first embodiment of a resistance heatingelement 10 includes a resistance wire 12 disposed in spiral circuitpath. The ends of the resistance wire 12 are coupled to a pair ofelectrical connectors 15 and 16 using known techniques such as,riveting, grommeting, brazing, clinching, compression fitting orwelding. The circuit includes a resistance heating material, which maybe a resistance heating wire 12 wound into a serpentine path containing,for example, about 3-200 windings, or, a resistance heating material,such as ribbon, a foil or printed circuit, or a conductive coating orink. Preferably the resistance heating wire 12 includes a Ni—Cr alloy,although certain copper, steel, and stainless-steel alloys could besuitable. A positive temperature coefficient wire may also be suitable.The resistance heating material can be provided in separate parallelpaths, or in separate layers. Whatever material is selected, it shouldbe electrically conductive, and heat resistant. It should also beresilient to subsequent forming operations, either on its owns, as inthe case of a wire or scrim, or encapsulated with a polymer. A tensilestrength of at least about 10,000 psi, and preferably at least about50,000 psi, for the fiber or resulting composite is helpful. (See ASTM D3379, D3039).

[0040] Alternatively, continuous or closed loop heating wires may beprovided, in which case current is induced into the heating element bymeans such as high frequency radiation or magnetic induction.

SUBSTRATES

[0041] As used herein, the term “supporting substrate” refers to thebase material on which the resistance material, such as wires, areapplied, or impregnated within, as is the case with graphite powder, forexample. The supporting substrate 11 of this invention should be capableof being pierced, penetrated, or surrounded, by a sewing needle forpermitting the sewing operation. Other than this mechanical limitation,the substrates of this invention can take on many shapes and sizes. Flatflexible substrates are preferably used for attaching an electricalresistance wire with a thread. Non-plastic materials, such as glasses,semiconductive materials, and metals, can be employed so long as theyhave a pierceable cross-sectional thickness, e.g., less than a 0.010inch-0.020 inch, or a high degree of porosity or openings therethrough,such as a grid, scrim, woven or non-woven fabric, for permitting thesewing needle of this invention to form an adequate stitch. Thesupporting substrate 11 of this invention need not necessarilycontribute to the mechanical properties of the final heating element,but may contain high strength fibers. Such fibers could contain glass,aramid fibers melt-bonded or joined with an adhesive to form a scrim,woven or non-woven mat.

[0042] Alternatively, the supporting substrate 11 of this invention maycontain ordinary, natural, or synthetic fibers, such as cotton, wool,silk, rayon, nylon, polyester, polypropylene, polyethylene, etc. Thesupporting substrate may also comprise a synthetic fiber such as Kevlarthat has good thermal uniformity and strength. The advantage of usingordinary textile fibers, is that they are available in many thicknessesand textures and can provide an infinite variety of chemistry, porosityand melt-bonding ability. The fibers of this invention, whether they beplastic, natural, ceramic or metal, can be woven, or spun-bonded toproduce non-woven textile fabrics.

[0043] Specific examples of supporting substrates 11 useful in thisinvention include polymer, ceramic, glass, or metallic films, such asnon-woven fiberglass mats bonded with an adhesive or sizing materialsuch as model 8440 glass mat available from Johns Manville, Inc.Additional substrates can include polymer impregnated fabric organicfabric weaves, such as those containing nylon, rayon, or hemp etc.,porous mica-filled plate or sheet, and thermoplastic sheet filmmaterial. In one embodiment, the supporting substrate 11 contains apolymeric resin which is also used in either the first thermoplasticsheet 110 or second thermoplastic sheet 105, or both of a heated elementassembly 100 described below. Such a resin can be provided in woven ornon-woven fibrous form, or in thin sheet material having a thickness of0.020 inch or less. Thermoplastic materials can be used for thesupporting substrate 11 which will melt-bond or liquefy with thethermoplastic sheets 110, 105, so as to blend into a substantiallyuniform structure.

SEWING OPERATION

[0044] With reference to FIG. 2, the preferred programmable sewingmachine 20 will now be described. The preferred programmable sewingmachine is one of a number of powerful embroidery design systems thatuse advanced technology to guide an element designer through designcreation, set-up and manufacturing. The preferred programmable sewingmachine 20 is linked with a computer 22, such as a personal computer orserver, adapted to activate the sewing operations. The computer 22preferably contains or has access to, embroidery or CAD software forcreating thread paths, borders, stitch effects, etc.

[0045] The programmable sewing machine 20 includes a series of bobbins24 for loading thread and resistance heating wire or fine resistanceheating ribbon. Preferably, the bobbins 24 are pre-wound to controltension since tension, without excessive slack, in both the top andbottom bobbins 24 is very important to the successful capturing ofresistance heating wire on a substrate. The thread used should be of asize recommended for the preferred programmable sewing machine. It musthave consistent thickness since thread breakage is a common mode offailure in using programmable sewing machines. An industrial qualitynylon, polyester or rayon thread is highly desirable. Also, a high heatresistant thread may be used, such as a Kevlar thread or Nomex threadknown to be stable up to 500° F. and available from Saunders Thread Co.of Gastonia, N.C.

[0046] The programmable sewing machine preferably has 1-20 heads and canmeasure 6 ft in width by 19 feet long. The sewing range of each head isabout 10.6 inches by 26 inches, and with every other head shut off, thesewing range is about 21 inches by 26 inches. An acceptable programmablesewing machine is the Tajima Model No. TMLG116-627W (LT Version) fromTajima, Inc., Japan.

[0047] The preferred method of capturing a resistance heating wire 12onto a supporting substrate 11 in this invention will now be described.First, an operator selects a proper resistive element material, forexample, Ni-Cr wire, in its proper form. Next, a proper supportingsubstrate 11, such as 8440 glass mat, is provided in a form suitable forsewing. The design for the element is preprogrammed into the computer 22prior to initiating operation of the programmable sewing machine 20. Aswith any ordinary sewing machine, the programmable sewing machine 20 ofthis invention contains at least two threads, one thread is directedthrough the top surface of the supporting substrate, and the other isdirected from below. The two threads are intertwined or knotted, ideallysomewhere in the thickness of the supporting substrate 11, so that onecannot view the knot when looking at the stitch and the resultingresistance heating element 10. As a top needle penetrates the substrate11 and picks up a loop of thread mechanically with the aid of themechanical device underneath, it then pulls it upward toward the centerof the substrate 11 and if the substrate is consistent and the threadtension is consistent, the knots will be relatively hidden. In apreferred embodiment of this invention, the resistance heating wire 12is provided from a bobbin in tension. The preferred programmable sewingmachine 20 of this invention provides a third thread bobbin for theelectrical resistance wire 12, so that the programmable sewing machine20 can lay the resistance wire 12, down just in front of the top needle.The preferred operation of this invention provides a zig zag or crossstitch pattern, whereby the top needle criss-crosses back and forth asthe supporting substrate 11 is moved, similar to the way an ornamentalrope is joined to a fabric in an embroidery operation. A simple loopingstitch with a thread 14 is also shown. By guiding the top needle overeither side of the resistance heating wire 12, the heating wire 12, iscaptured in a very effective manner, the process being computercontrolled so that the pattern can be electronically downloaded into thecomputer 22 and automatically sewn onto a substrate of choice.

[0048] The programmable sewing machine 20 can sew an electricalresistance heating wire 12 having a diameter or thickness of 0.005 inch-0.25 inch, onto a supporting substrate 11 at a rate of about 10-500stitches per minute, saving valuable time and associated cost in makingresistance heating elements.

[0049] The ability to mechanically attach resistive elements, such aswires, films and ribbons, to substrates provides a multitude of designpossibilities in both shape and material selection. Designers may mixand match substrate materials by selecting their porosity, thickness,density and contoured shape with selected resistance heating materialsranging in cross-section from very small diameters of about 0.005 inchto rectangular and irregular shapes, to thin films. Also, secondarydevices such as circuits, including microprocessors, fiberoptic fibersor optoelectronic devices, (LEDs, lasers) microwave devices (poweramplifiers, radar) and antenna, high temperature sensors, power supplydevices (power transmission, motor controls) and memory chips, could beadded for controlling temperature, visual inspection of environments,communications, and recording temperature cycles, for example. Theoverall thickness of the resistance heating element is merely limited bythe vertical maximum position of the needle end, less the wire feed,which is presently about 0.5 inch, but may be designed in the future tobe as great as 1 inch or more. Resistive element width is not nearly solimited, since the transverse motion of the needle can range up to onefoot or more.

[0050] The use of known embroidery machinery in the fabrication ofresistance heating elements allows for a wide variety of raw materialsand substrates to be combined with various resistance heating materials.The above construction techniques and sewing operation also provide theability to manufacture multi-layered substrates, including embeddedmetallic and thermally conductive layers with resistance wires wrappedin an electrically insulating coating, so as to avoid shorting ofelectric current. This permits the application of a resistance heatingwire to both sides of the thermally conductive metallic layer, such asaluminum foil, for more homogeneously distributing resistance heat.

THERMOPLASTIC LAMINATE HEATING ELEMENT ASSEMBLY AND HEATING SHELFCONSTRUCTION

[0051]FIG. 3 shows an exemplary heating element assembly 100, in theform of a heating shelf, according to the invention. The heating elementassembly 100 includes a resistance heating element 10 disposed betweenlaminated first and second thermoplastic sheets 105, 110. Forillustrative purposes, the first thermoplastic sheet 105 is shownpartially removed from the second thermoplastic sheet 110. Theresistance heating element 10, described above, at least substantiallyencompasses the circuit path, defined by resistance wire 12.

[0052] The supporting substrate of the resistance heating element 10 hasa thickness between 0.005 inch and 0.25 inch, and is preferably 0.025inch thick. The supporting substrate should be flexible, either underambient conditions or under heat or mechanical stress, or both. A thinsemi-rigid heating element assembly 100 allows for closer proximity ofthe resistance heating wire 12 to an object to be heated when theheating element assembly is formed into a final element assembly, suchas a heating shelf. Because less heat needs to be generated by theresistance heating element 10 to provide heat to the outer surfaces of athin heating element assembly 100, materials having lower RTI (RelativeThermal Index) ratings can be successfully used in thin heating elementassemblies.

[0053] The thermoplastic sheets 105, 110 are laminated to each other tosecure resistance heating element 10 and to form a reformable continuouselement structure. The thermoplastic sheets 105, 110 may be heated andcompressed under sufficient pressure to effectively fuse thethermoplastic sheets together. A portion of this heat may come fromenergizing the resistance heating element 10. Alternatively,thermosetting polymer layers could be employed, such as B-stage epoxysheet or pre-preg material.

[0054] Preferred thermoplastic materials include, for example:fluorocarbons, polypropylene, polycarbonate, polyetherimide, polyethersulfone, polyaryl-sulfones, polyimides, and polyetherkeytones,polyphenylene sulfides, polyether sulfones, and mixtures and co-polymersof these thermoplastics. An acceptable thermoplastic polyetherimide isavailable from the General Electric Company under the trademark ULTEM.

[0055] It is further understood that, although thermoplastic materialsare preferable for forming fusible layers because they are generallyheat-flowable, some thermoplastics, notably polytetraflouroethylene(PTFE) and ultra high-molecular-weight polyethylene (UHMWPE) do not flowunder heat alone. Also, many thermoplastics are capable of flowingwithout heat, under mechanical pressure only.

[0056] Acceptable results were achieved when forming a heating elementassembly under the conditions indicated in TABLE 1 as follows: TABLETHICKNESS OF SHEET PRESSURE TIME TEMP. MATERIAL (inch) (PSI) (minutes)(° F.) Polypropylene 0.009 22 10 350 Polycarbonate 0.009 22 10 380Polysulfone 0.019 22 15 420 Polyetherimide 0.009 44 10 430Polyethersulfone 0.009 44 10 460

[0057] Where no vacuum was applied, “thickness” is the thickness of thethermoplastic sheets in inches, “pressure” represents the amount ofpressure (in psi) applied to the assembly during lamination,“temperature” is the temperature applied during lamination, and “time”is the length of time that the pressure and heat were applied. It willbe understood the above-identified material thicknesses used in formingexemplary embodiments of the assembly described herein are merelyprovided by way of example. Materials of differing thicknesses may alsobe used to achieve acceptable results without departing from the scopeof the invention.

[0058] The first and second thermoplastic sheets 105, 110 and resistanceheating element 10 of the heating element assembly 100 may also belaminated to each other using an adhesive. In one embodiment of thepresent invention, an adhesive to hold the materials together, which maybe an ultraviolet curable adhesive, may be disposed between theresistance heating element 10 and the first thermoplastic sheet 105 andbetween the resistance heating element 10 and the second thermoplasticsheet 110, as well as between areas of the thermoplastic sheets 105, 110which are aligned to be in direct contact. An ultraviolet curableadhesive may be used that is activated by ultraviolet light and thenbegins to gradually cure. In this embodiment of the present invention,the adhesive may be activated by exposing it to ultraviolet light beforeproviding the second of the thermoplastic sheets 105, 110. Thethermoplastic sheets 105, 110 may then be compressed to substantiallyremove any air from between the sheets 105, 110 and to secure resistanceheating element 10 therebetween.

[0059]FIG. 5 illustrates that a heating element assembly 100 a mayinclude a plurality of heated layers. A second resistance heatingelement 10 a may be laminated between one of thermoplastic sheets105,110 and a third thermoplastic sheet 115.

[0060] The thicknesses of thermoplastic sheets 105, 110 and thethickness of supporting substrate 11 and resistance heating wire 12 arepreferably selected to form a reformable continuous element structurethat maintains its integrity when the element is formed into a finalelement structure. The preferred heating element assembly 100 accordingto the invention, then, is a semi-rigid structure in that it may bereformed, such as by simply molding, folding or unfolding under heat,pressure, or a combination thereof as required by the chosenthermoplastics, into a desired shape without sacrificing structuralintegrity.

[0061] Heating shelves 100 according to the present invention provideseveral advantages over non-rigid and rigid shelves or containers, whichdo not include a heat source. The heat source, i.e., the resistanceheating element 10, intimately surrounds the contents of a shelf 100,which may be, for example, a food product such as cookies, muffins,donuts, pizza, sandwiches, or other contents, whether they be solid,semi-solid or liquid. Also, secondary devices as described above, suchas temperature gauges, sensors, thermocouples, and RTD's may be disposedmore intimately with the contents or conditions that are beingmonitored.

[0062] A heating shelf 100 may also be positioned in a mold, overmolded, or both, to form a selected molded heated structure. Someplastics may be energized prior to and/or during over molding forimproved bonding with the over molding material. A heating shelf 100 mayoptionally be thermoformed to conform to at least a part of the moldstructure and to preferentially align the resistance heating elementwithin the mold. Once the heating shelf is positioned within a mold, theresistance heating element 10 of the heating shelf 100 may be energizedto soften the thermoplastic sheets, and the heating shelf may be overmolded with a thermoplastic. The energizing and overmolding may be timedsuch that the thermoplastic sheets and over molded thermoplastic form asubstantially homogenous structure when solidified. Alternatively, thethermoplastic sheets may be allowed to soften as a result of mold flowalone. The thermoplastic materials of the sheets and over moldedthermoplastic are preferably matched to further facilitate the creationof a homogenous structure. The supporting substrate 11 may also beselected to be a thermoplastic to better promote the formation of ahomogenous structure. The energizing may be timed to soften thethermoplastic sheets before, after, or during the overmolding process,depending upon the standard molding parameters, such as the flowcharacteristic of the selected thermoplastics, the injection moldingfill time, the fill velocity, and mold cycle. The assembly is alsoamenable to other molding processes, such as injection molding,compression molding, thermoforming, and injection-compression molding.

[0063]FIGS. 8 and 9 illustrate an exemplary heating element assembly,which may be formed into a heating shelf 100 final element assembly.FIG. 8 is a top plan view of an exemplary resistance heating element400. The resistance heating element 400 includes a supporting substrate405 shaped in the profile of a flattened container. The profile mayeither be initially shaped in this profile shape or cut to the profileshape from a larger supporting substrate. Resistance heating material isaffixed to the supporting substrate 405 and is preferably resistancewire 410 sewn to supporting substrate 405.

[0064] The resistance heating element 400 shown in FIG. 8 includes aplurality of flap portions 420 capable of rotation about a first axis ofrotation indicated generally at fold lines 425. The circuit path 415formed by resistance wire 410 terminates at terminal end portions 412.

[0065]FIG. 9 is a top plan view of a heating element assembly 500. Theresistance heating element 400 is laminated between two thermoplasticsheets, only the top sheet 505 of which is shown, to form a reformablecontinuous element structure. A portion of the thermoplastic sheet 505is shown removed in order to show the resistance heating element 400.

[0066] The dashed lines 530 indicate portions of the laminated structurethat may be removed, such as by stamping or die cutting, from thelaminated structure to leave a foldable profile which may be formed intoa non-planar shelf. Alternatively, the heating shelf may be formedwithout foldable flap portions. The remaining dashed lines of FIG. 9indicate fold lines. Other alternatives may include integrally forminggeometry features, which facilitate the assembly of the heating shelfwith existing display cabinet configurations.

[0067] A heating shelf 100 may be formed by folding the heating element500 along the dashed lines of FIG. 9 and in the direction of the arrowsshown in FIG. 3. The flaps 420 of the resistance heating element 400 arelaminated between thermoplastic layers and are folded into the shelfshape shown in FIG. 3. The folding step may include rethermalizing thethermoplastic structure while folding in order to thermoform thestructure into the desired heat planes, or, alternatively, folding thethermoplastic structure into the desired heat planes and thenrethermalizing the structure, although it is recognized that the lattermethod introduces residual stresses in the bend areas. The heating shelf100 may optionally be formed with outwardly flared sides. This featurepermits multiple shelves to be stacked in nested engagement, whichreduces spatial requirements for both storage and shipping.

[0068] It should be apparent that the heating shelf 100 can optionallyprovide heat on five different interior planes may, but is formed froman easily manufactured planar heating element 500. It should further beapparent that the present invention is not limited in any way to theheating shelf configuration 100 or heating element 500 described above.Rather, the above describe method of manufacturing and heating elementstructure may be used to forms cups, enclosed containers, boxes, or anyother structure which may be formed from a planar profile. The heatingshelves and other configurations can include planar elements made fromresistance heating wires, scrim, woven and nonwoven fabric andconductive filing such as conductive polymers, inks and foils. Suchplanar forms should have sufficient tensile strength to resistmechanical distortion of the circuit path, or heater distributionprofile, during forming of the final product.

[0069] A sheet of heating element assemblies and a method ofmanufacturing the same is described hereafter. In another exemplaryembodiment of the present invention, a sheet of heating elementassemblies 225 is provided, as shown in FIG. 6. The sheet of heatingelement assemblies 225 includes first and second affixed thermoplasticsheets, as described above, and a sheet of resistance heating elements200 (FIG. 7) secured between and to the first and second thermoplasticsheets. Essentially, the sheet of resistance heating elements 200comprises a plurality of connected resistance heating elements 10. Thesheet of resistance heating elements 200 comprises a supportingsubstrate 205 and a plurality of spaced circuit paths 207, each of thecircuit paths 207 comprising an electrical resistance heating materialattached to the supporting substrate 205 to define a circuit path, whichincludes a pair of terminal end portions 208, 209. The shape of thecircuit paths 207 is merely illustrative of circuit path shapes, and anycircuit path shape may be chosen to support the particular end use for aheating element assembly included in the sheet of heated elementassemblies 225. Alternatively, conductive polymers or fabrics made fromresistance heating material could be employed. The dashed lines of FIG.7 indicate where an individual resistance heating element may be removedfrom the sheets of resistance heating elements 225.

[0070] A sheet 225 of heating element assemblies may be manufacturedusing conventional mass production and continuous flow techniques, suchas are described in U.S. Pat. No. 5,184,969 to Sharpless et al., theentirety of which is incorporated herein by reference. For example, asillustrated in FIG. 6, first and second thermoplastic sheets 210, 212may be provided from a source, such as rolls 214, 216 of thermoplasticsheets, or extruded using known extrusion techniques as a part of themanufacturing process. One manufacturer of such thermoplastic sheetextruders is Killion Extruders Inc. of Cedar Grove, N.J. Likewise, asheet of resistance heating elements 200 may be provided from a source,such as roll 218. Sheet 200 may be manufactured as described above inthe “Sewing Operation” section. The sheets 200, 212, 214 may be made toconverge, such as by rollers 224, between a heat source, such as radiantheating panels 220, to soften the thermoplastic sheets 210, 212. Aseries of rollers 222 compresses the three sheets 200, 212, 214 into asheet of heated element assemblies 225, thereby also removing air frombetween the sheets 200, 212, 214. The rollers 222 may also provide heatto help fuse the sheets 200, 212, 214 and/or may be used to cool freshlylaminated sheets 200, 212, 214 to help solidify the heated sheets intothe sheet of heated element assemblies 225 after compression.

[0071] It should be apparent that a sheet of a plurality ofmultiple-layered heating element assemblies, such as a sheet including aplurality of heating element assemblies 100 a of FIG. 5, may also bemanufactured simply by including a third thermoplastic sheet and asecond sheet of resistance heating elements to the process describedabove.

[0072] Regardless of the specific manufacturing technique, the sheet ofheating element assemblies 225 may be collected into a roll 230. Theroll 230 may then be used by an original equipment manufacture (OEM) forany desired manufacturing purpose. For example, the OEM may separate orcut individual heating element assemblies from the roll and include theheating element assembly in a desired product, by molding, adhesive orultrasonic bonding, for example, into a container or molded product. Anindividually manufactured heating element assembly as mentioned above ora heating element assembly removed from a sheet of heating elementassemblies 225, because of its resiliency and good mechanicalproperties, is amenable to secondary manufacturing techniques, such asdie cutting, stamping, or thermoforming to a desired shape orcombination thereof as described above. Each heating element assemblymay be cut or stamped into a preselected shape for use in a particularend product even while still a part of sheet 225 and then collected intoa roll 230. The circuit paths of the resistance heating element of theheating element assembly may be appropriately shaped to conform to thedesired shape of a selected product and heat planes in which the heatingelement assembly is to be included or formed.

[0073] The formable semi-rigid feature of the heating element assembliesof the present invention provides a designer the opportunity to includethe assembly in complex heat planes. The assembly may be cut to adesired formable shape, and the circuit path is preferably designed tosubstantially conform to this shape or the desired heat planes. Theassembly may then be rethermalized and folded to conform to the heatplanes designed for the assembly to occupy.

[0074] A preferred thermoplastic sheet may range from approximately0.004 inch to 0.100 inch. Thus, the thickness of the thermoplasticsheets of a heating element assembly may be chosen to effectively biasheat generated by a resistance heating element in a selected direction.The supporting substrate itself also may provide an insulation barrierwhen the circuit path is oriented towards, for example, contents to beheated and the supporting substrate is oriented toward an outer orgripping surface.

[0075] Similarly, one or both of the thermoplastic sheets of a heatingelement assembly 100 or heating element assembly 500 may be coated witha thermally conductive coating that promotes a uniform heat plane on theheated element assembly. An example of such a coating may be found onanti-static bags or Electrostatic Interference (ESI) resistive bags usedto package and protect semiconductor chips. Also, thermally conductive,but preferably not electrically conductive, additive may be added to thethermoplastic sheets to promote heat distribution. Examples of suchadditive may be ceramic powders, such as, for example, Al₂O₃, MgO, ZrO₂,boron nitride, silicon nitride, Y₂O₃, SiC, SiO₂, TiO₂, etcetera. Athermally conductive layer and/or additive is useful because aresistance wire typically does not cover all of the surface area of aresistance heating element 10.

[0076] Advantageously, a heating assembly, formed in accordance with theinvention, may be provided having varying surface watt densities, toprovide for accurate heat placement. Other alternatives includeproviding a heating shelf having a plurality of resistance heatingelements, in which case one element could be used for initialtemperature boosting, while a second resistance element could be usedfor maintenance heating.

EXPERIMENTAL RESULTS

[0077] A heating shelf was formed having a resistance heating circuitpath sandwiched between laminated layers of thermoplastic. Thethermoplastic material used for both the top and bottom of the heatingshelf assembly was ULTEM 1000. The top of the heating shelf was formedwith two sheets of ULTEM 1000 having a total thickness of 0.02 inch. Thebottom of the heating shelf was formed from laminated sheets having atotal thickness of 0.095 inch. It will be understood that materials usedin forming the heating shelf are not limited to the precise thicknessesdefined herein, which are merely provided by way of example. Aresistance heating circuit path was formed using resistance heating wirehaving a total impedance of approximately 289 ohms. The resistanceheating wire may comprise a plurality of twisted, braided or parallelindividual wires having a collective diameter of between about 0.010inch to 0.050 inch. The resistance heating wire was sewn to a fiberglassscrim substrate having an uncompressed thickness of approximately 0.030inch. It will be understood that materials used in forming the heatingshelf are not limited to the precise thicknesses defined herein, whichare merely provided by way of example. The resistance heating wire waspatterned in a spiral design starting in the center of the shelf with ½inch spacing, which is progressively reduced to ¼ inch.

[0078] The substrate, having a resistance heating wire sewn thereto, wasplaced between the top and bottom thermoplastic sheets to form a heatingelement assembly. Next, the heating element assembly was sandwiched in amanufacturing assembly. To this end, a Teflon sheet was placed adjacentto the exposed surface of each thermoplastic sheet, a layer of siliconrubber was placed adjacent each Teflon sheet, and a stainless steelplate was placed adjacent each silicon rubber sheet. The Teflon preventsthe thermoplastic sheets from adhering to the manufacturing assembly,while the silicon rubber sheets provide a cushion which allows for evendistribution of the hydraulic pressure applied by the heat press. Thestainless steel sheets act as stiffening agents to facilitate handlingof the otherwise pliable assembly.

[0079] The resulting manufacturing assembly was then placed in aconventional heated press, with temperature platens preheated to 450degrees Fahrenheit. The assembly was heated for 20 minutes at a pressureof 20,000 lbs. The assembly was then air cooled for 20 minutes, followedby a 2 minute water cooling period. The heater was then trimmed to finaldimensions using a belt sander.

[0080] After forming and cooling the heating element assembly, theassembly was reheated along bend lines, about which the two flapportions were folded to reform the assembly into a heating shelf.

[0081] A performance graph for the above-described heating shelf isshown in FIG. 10. The heating shelf was placed on two laterally spacedwood strips, each having a width 0.75 inch. The baked cookies fortesting, packaged in pairs in polyethylene bags, were placed on theheating shelf and warmed to a desired serving temperature. The cookieswere then removed from the shelf.

[0082] The performance graph shows that the cookie temperature at thecenter of the shelf stabilized at 133 degrees Fahrenheit, and the cookietemperature at the edge stabilized at 128 degrees Fahrenheit. The loadedheater temperature was 155 degrees Fahrenheit. After the cookies wereremoved, the heater stabilized at 124 degrees Fahrenheit.

ADVANTAGES OF THE INVENTION

[0083] A heating shelf in accordance with the invention provides moreefficient heating of food products. Indeed, experimental results haveshown that the present invention consumes ⅓ less wattage thantraditional heating methods. This significant power savings isattributed in part to the intimate contact achievable between theheating shelf and the food product as compared to conventional heatingmethods. Another factor attributing to improved heating efficiency isthe ability to design and manufacture the product with a varied wattdensity, thereby allowing the accurate placement of heat such that thefood product can evenly warmed throughout, while preventing over warmingof food product.

[0084] Also, the heating shelf is hermetically sealed, making the shelfsuitable for direct contact with food products, and allowing for theutilization of conventional cleaning techniques such as dishwashersetcetera, without compromising the integrity of the shelf.

[0085] Yet another advantage of the invention is the thin yet rigidshelf geometry for more efficient utilization of existing cabinet space.

[0086] The preferred heating shelf has an operating voltage of 120 Vac,thereby making the heating shelf mobile as compared to other comparabledevices requiring 240 Vac supply source.

[0087] Further, as described above, the heating shelf of the presentinvention lends itself to many automated and secondary manufacturingtechniques, such as stamping, die cutting, and overmolding, to name afew. Designers can easily choose thermoplastics and other materials fortheir designs that meet required RTI (relative thermal index)requirements for specific applications by following standard designtechniques and parameters set by materials manufacturers Also, heatingshelves such as described above allow the food industry to efficientlyand effectively reheat prepared foods, as is often required ofbusinesses that operate large or small food service venues or thatpurchase from distributors of prepared foods. Also, among the manyadvantages of the present invention is the ability to intimately locatea secondary device captured between the thermoplastic sheets, such as amemory device or other data collector within close proximity to a foodproduct, thereby allowing more accurate data collection, such asdisclosed in commonly owned U.S. Pat. No. 6,417,335, herein incorporatedin its entirety by reference. This data, as an example, may be used toprove that a food was prepared at a temperature and for a time periodsufficient to kill the E. coli bacteria.

[0088] Although various embodiments have been illustrated, this is forthe purpose of describing, but not limiting the invention. The assemblyline described above is merely illustrative of one means of forming asheet of heated element assemblies. Further, the supporting substrateshapes and circuit paths described above and shown in the drawings aremerely illustrative of possible circuit paths, and one of ordinary skillshould appreciate that these shapes and circuit patterns may be designedin other manners to accommodate the great flexibility in uses and numberof uses for the heating element assembly of the present invention.Therefore, various modifications which will become apparent to oneskilled in the art, are within the scope of this invention described inthe attached claims.

We claim:
 1. A method of manufacturing a heating shelf, comprising thesteps of: (a) disposing at least one resistance heating element betweenfirst and second thermoplastic sheets, at least of the thermoplasticsheets having a visible feature that changes with temperature, each ofthe at least one resistance heating elements comprising: (i) asupporting substrate; and (ii) an electrical resistance heatingmaterial, wherein the electrical resistance heating material is one ofattached to and supported in the substrate, the electrical resistanceheating material forming a circuit path; (b) laminating the first andsecond thermoplastic sheets such that each of the at least oneresistance heating element is secured between the first and secondthermoplastic sheets to form a reformable structure; and (c) forming thestructure into a heating shelf.
 2. The method of claim 1 wherein the atleast one thermoplastic sheet, having a visible feature that changeswith temperature, is one of a thermochromic material and an LEDindicator.
 3. The method of claim 1 wherein each heating element furthercomprises: at least one flap portion, capable of rotation about a firstaxis of rotation, at least one of the circuit paths continuing onto theflap portion, wherein the step of forming includes rotating the flapportion about the first axis to provide resistance heating in at leasttwo planes.
 4. The method of claim 1, wherein said step of laminatingincludes the steps of heating said thermoplastic sheets and compressingsaid thermoplastic sheets to laminate the resistance heating elementsbetween the thermoplastic sheets.
 5. The method of claim 1, wherein saidstep of forming includes the step of thermoforming the reformablestructure into the heating shelf, whereby said supporting substrate andelectrical resistance material resist forces which are capable ofbreaking or shorting said circuit path.
 6. The method of claim 1,further comprising the step of cutting the continuous element structureinto a foldable profile before forming the continuous reformablestructure into the heating shelf.
 7. The method of claim 1, furthercomprising the steps of: (d) energizing at least one of the resistanceheating elements to soften the thermoplastic sheets; and (e) overmoldingthe heating shelf with a thermoplastic, the steps of energizing andovermolding timed such that the thermoplastic sheets and over moldedthermoplastic form a substantially homogenous structure.
 8. A method ofmanufacturing a heating shelf, comprising the steps of: (a) disposing atleast one resistance heating element between first and secondthermoplastic sheets, the at least one resistance heating elementcomprising: (i) a supporting substrate; and (ii) at least one circuitpath, each of the circuit paths comprising an electrical resistanceheating material attached to the supporting substrate, at least one ofthe circuit paths having terminal end portions, at least one of thecircuit paths continuing onto a first flap portion of the substratecapable of rotation about a first axis of rotation; and (b) laminatingthe first and second thermoplastic sheets such that the at least oneresistance heating element is secured between the first and secondthermoplastic sheets; (c) attaching a material having a visible featurethat changes with temperature to the heating shelf.
 9. The method ofclaim 8 wherein the material having a visible feature that changes withtemperature is one of a thermochromic material and LED indicator. 10.The method of claim 8 wherein the step of attaching comprises laminatingthe material having a visible feature that changes with temperature tothe heating shelf.
 11. The method of claim 9, wherein the thermochromicmaterial is disposed between the first and second thermoplastic sheets.12. A method of manufacturing a sheet of heating element assemblies,comprising the steps of: (a) disposing at least one sheet of resistanceheating elements between first and second thermoplastic sheets, at leastone of the thermoplastic sheets having a visible feature that changeswith temperature, each of the resistance heating elements attached to asupporting substrate and forming a circuit path, at least one of thecircuit paths having terminal end portions, at least one of the circuitpaths continuing onto a first flap portion of the substrate capable ofrotation about a first axis of rotation; and (b) laminating the firstand second thermoplastic sheets such that the at least one sheet ofresistance heating elements is secured between the first and secondthermoplastic sheets to form a reformable structure.
 13. The method ofclaim 12 wherein the at least one thermoplastic sheet having a visiblefeature that changes with temperature, is one of a thermochromicmaterial and LED indicator.
 14. The method of claim 12, furthercomprising the steps of removing at least one heating element assemblyfrom the sheet of heating element assemblies, the removed heatingelement assembly being a reformable structure, and forming thereformable structure into a final element assembly configuration whereinat least the first flap portion of the substrate is rotated about thefirst axis to provide resistance heating in at least two planes.
 15. Themethod of claim 12, further comprising the steps of cutting at least oneof the heating element assemblies into a foldable profile before formingthe reformable structure into the final element assembly configuration.16. The method of claim 15, wherein said step of cutting includes thestep of stamping or die cutting at least one of the heating elementassemblies into the profile.
 17. A heating element assembly, comprising:(a) a first thermoplastic sheet; (b) a second thermoplastic sheet, atleast one of the thermoplastic sheets having a visible feature thatchanges with temperature; and (c) a resistance heating element securedbetween the first and second thermoplastic sheets, the resistanceheating element being attached to a supporting substrate and forming aat least one circuit path having terminal end portions, at least one ofthe circuit paths continuing onto a first flap portion of the substratecapable of rotation about a first axis of rotation, wherein thethermoplastic sheets and resistance heating element are laminatedtogether to form a reformable structure, the reformable structure formedinto a final element assembly configuration wherein at least the flapportions is rotated about the first axis to provide resistance heatingin at least two planes.
 18. The heating element assembly of claim 17wherein the at least one thermoplastic sheet having a visible featurethat changes with temperature, is one of a thermochromic material andLED indicator.
 19. The heating element assembly of claim 18, wherein thethermoplastic sheets are affixed with an adhesive.
 20. The heatingelement assembly of claim 18, wherein the thermoplastic sheets areattached by one of fusing and laminating.
 21. The heating elementassembly of claim 17, wherein the reformable structure is thermoformedinto said final element assembly configuration.
 22. The heating elementassembly of claim 17, wherein the reformable structure is cut into afoldable profile.
 23. The heating element assembly of claim 17, whereinthe electrical resistance heating material is at least one of glued,sewn and fused to the supporting substrate.
 24. The heating elementassembly of claim 17, wherein the electrical resistance heating materialis sewn to said supporting substrate with a thread.
 25. The heatingelement assembly of claim 17, wherein the supporting substrate comprisesat least one of a woven and non-woven fibrous layer.
 26. The heatingelement assembly of claim 17, wherein the supporting substrate is athermoplastic sheet.
 27. The heating element assembly of claim 17,wherein the supporting substrate includes thermally conductiveadditives.
 28. The heating element assembly of claim 17, wherein atleast one of the thermoplastic sheets includes a thermally conductivecoating.
 29. The heating element assembly of claim 17, furthercomprising a secondary device secured between the first and secondthermoplastic sheets.
 30. The heated element assembly of claim 17,wherein one of the thermoplastic sheets is thicker than the otherthermoplastic sheet.
 31. The heating element assembly of claim 17,wherein the heating element assembly is over molded with a thermoplasticsuch that the over molded thermoplastic and thermoplastic sheets form asubstantially homogenous structure.
 32. The heating assembly of claim17, wherein at least one circuit path is a continuous loop, which iscapable of being energized by at least one of high frequency radiationand magnetic induction.
 33. The heating assembly of claim 28, whereinthe secondary device is one of, a thermistor, a sensor and athermocouple.
 34. The heating assembly of claim 17, wherein at least oneof the thermoplastic sheets is Polyetherimide.
 35. The heating assemblyof claim 17 wherein the final element assembly is hermetically sealed.36. The heating assembly of claim 17, wherein the heating elementassembly has a bottom and the circuit path density in the bottom of theheating element assembly is greater than the circuit path density in theflap portions.
 37. The heating assembly of claim 17, wherein the flapportions are outwardly flared to provide for nested engagement with asecond identical heating element assembly.
 38. A method of manufacturinga sheet of heating element assemblies, comprising the steps of: (a)disposing at least one sheet of resistance heating elements betweenfirst and second thermoplastic sheets, the at least one sheet ofresistance heating elements being attached to a supporting substrate andforming a plurality of spaced apart circuit paths each of the circuitpaths having terminal end portions and each of the circuit pathscontinuing onto a first flap portion of the substrate capable ofrotation about a first axis of rotation, wherein the thermoplasticsheets and resistance heating element are laminated together to form areformable structure, the reformable structure formed into a finalelement assembly configuration where the flap portion is rotated aboutthe first axis to provide resistance heating in at least two planes. (i)a supporting substrate; and (ii) at least one circuit path, each of thecircuit paths comprising an electrical resistance heating materialattached to the supporting substrate, at least one of the circuit pathshaving terminal end portions, at least one of the circuit pathscontinuing onto a first flap portion onto a first flap portion of aresistance heating element capable of rotation about a first axis ofrotation; and (b) disposing a sheet of material having a visible featurethat changes with temperature between the first and second thermoplasticsheets. (c) attaching the first and second thermoplastic sheets suchthat the at least one sheet of resistance heating elements is securedbetween the first and second thermoplastic sheets to form a continuouselement structure, wherein the first and second thermoplastic sheets andresistance heating elements are laminated such that the sheet ofresistance heating elements is secured between the first and secondthermoplastic sheets to form a reformable structure.
 39. The method ofclaim 38 wherein the sheet of material having a visible feature thatchanges with temperature, is thermochromic.
 40. The method of claim 38wherein the sheet of heating element asemblies further compries anadhesive attaching said first and second thermoplastic sheets. changeswith temperature, is thermochromic.
 41. The method of claim 38 whereinthe first and second thermoplastic sheets are attached by one of fusingand laminating.
 42. The method of claim 38 wherein the electricalresistance heating material is at least one of glued, sewn and fused tothe supporting substrate.
 43. The method of claim 38 wherein saidelectrical resistance heating material is sewn to said supportingsubstrate with a thread.
 42. The method of claim 38 wherein thesupporting substrate comprises at least one of a woven and non-wovenfibrous layer.
 43. The method of claim 38, wherein the supportingsubstrate is an extruded thermoplastic sheet.
 44. The method of claim 38wherein the heating element assembly further comprises a plurality ofsecondary devices, each of said secondary devices disposed between saidfirst and second thermoplastic sheets and associated with one of saidcircuit paths.
 45. The method of claim 38 wherein at least one of thethermoplastic sheets includes a thermally conductive coating.
 46. Aheating shelf, comprising: (a) a first thermoplastic sheet; (b) a secondthermoplastic sheet; (c) a resistance heating element disposed betweenthe first and second thermoplastic sheets, the resistance heatingelement comprising: (i) a supporting substrate including at least onecircuit path, comprising an electrical resistance heating materialattached to, or disposed within, the supporting substrate, said circuitpath having terminal end portions and continuing onto the flap portionof the substrate; and (d) a material having a visible feature thatchanges with temperature attached to the heating shelf, wherein thethermoplastic sheets and said electrical resistance heating material arelaminated together to form a reformable structure, the reformablestructure formed into a final element assembly wherein the flap portionis rotated about the first axis to provide resistance heating in atleast two planes.